Various correction systems have been used to correct spinal deformities. For example, the Harrington system, introduced in the early 1950s, employs a distraction rod on the concave side of the deformed spine. The use of the Harrington system and spinal fusion in treating idiopathic scoliosis was found useful for both single and double thoracic contours. However, a loss of lumbar lordosis, or the “flatback” effect, can occur when using the Harrington system along with distraction over the lumbar spine. This effect could be countered by segmental wiring (e.g., sublaminar wiring or wiring of the spinous process) over the instrumented site to retain the lumbar lordosis. Nevertheless, the Harrington system can have limited control of sagittal plane correction, limited derotation, and high rate of hook dislodgment in the lumbar spine, encounter distraction rod fracture, and require postoperative external support.
Armstrong and Connock, and Cotrel and Associates introduced the use of a compression rod on the convex side, which uses transverse traction with Harrington rods to correct deformities. Luque and Jacobs introduced the use of strengthened upper hooks to prevent pull out and increase yield strength of the distraction rod. These configurations helped the Jacobs's system achieve correction in hyperextension.
The Luque system employs a spinal implant for scoliosis correction. The use of the spinal implant involves both convex technique and concave technique. The convex technique is usually used to treat patients with thoracic curve, whereas the concave technique is used on patients with lumbar curve or severe deformity. The Luque system uses sublaminar wiring as multiple segmental fixation points with attachment to a rod and the L-shaped Luque rod. The sublaminar wiring provides fixation at various points along the instrumented area and allows the correction forces to distribute along the spine, thereby lowering the possibility of bone fracture and the need for post-operative immobilization.
The C-D spinal instrumentation system, introduced by Cotrel and Dubousset, employs a dual rod system interlinked by a transverse traction device (DTT), and multiple hooks on each rod. The C-D system was intended to improve thoracic lordosis, preserve lumbar lordosis in the sagittal plane, improve correction in the frontal plane, and minimize loss of correction in case of hook migration or fracture at the bone-metal interface.
Similar spinal correction systems include the Texas Scottish Rite Hospital (TSRH) Universal Spinal Instrumentation and Isola spinal implant system. The TSRH system, in addition to obtaining a better correction of thoracic curves in the sagittal and coronal planes and maintaining lumbar lordosis, also provides rigidity of the implant against axial and torsional forces with its Crosslink™ device. The TSRH system has been used to treat severe scoliotic curves. The Isola spinal implant system was developed from Harrington's principles and designs and assembled with variable screw placement system (VSP). The assembly of this implant is intended to minimize internal or external profiling and increase stability and durability. However, complications associated with iliac screw breakage, transverse connector breakage and screw breakage at the end of constructs have occurred when using the Isola system.
The various conventional correction systems however fail to provide complete correction of scoliotic spines. For example, conventional spinal correction systems, from the earlier Harrington, Jacobs and Luque systems to the later developed systems such as TSRH (SOFAMOR DANEK, US), CD Horizons (SOFAMOR DANEK, US) and ISOLA (DePuyAcroMed, Raynbam, Mass., US) and Moss Miami (DePuy AcroMed), use slightly different techniques to correct scoliosis. Such correction systems can reduce spinal deformity by only 60% to 70%, but a full correction is almost impossible. The use of excessive correction forces in attempting a full correction can cause bony fractures or neurological deficit due to spinal cord damage.
Moreover, none of the conventional correction systems takes into account the viscoelastic behavior of the spine. Viscoelastic properties of the spine relate to its time dependent mechanical effect, i.e., the stiffness of spine decreases over the duration for which the force is applied. Such mechanical effect has been observed in spinal surgery, where the force required to hold tissue in tension decreases gradually during the operation. The correction force applied by the correction system will also decrease as the tissues relax. This loss of tension within the correction systems can cause a partial recurrence of deformity subsequent to the correction. Consequently, conventional correction systems can cause a loss of correction for up to 15% as a result of this effect. Furthermore, because scoliosis correction is carried out instantaneously and only at the time of surgery with no time for the tissues to relax, the load on the spine rises rapidly. The increased load on the corrected spine limits the amount of correction force that is safely possible. Thus, viscoelastic behavior of spinal tissues can limit the amount of correction force and in turn the correction rate. If excessive correction forces are used, they can cause bony fractures. In addition, neurological deficit can occur due to rapid over-stretching of the spinal cord.
Gradual and constant correction forces have been adopted to overcome the deficiency caused by viscoelastic relaxation of spinal tissues. Such correction forces can take up the loss of correction occurred due to the viscoelastic behaviors of biological tissues and effect a gradual correction of the deformity after the initial surgery. Conventional systems providing gradual or constant correction forces nevertheless require some form of repeated surgery or post-operative care, resulting in longer hospitalization as well as patient inconvenience and discomfort.
Other methods in which a gradual but constant force is applied to the scoliotic spine include external halo traction, intermittent open lengthening and shape change from shape memory alloys. For example, the shape memory effect (SME) of nickel-titanium (NiTi) alloy has been used to correct scoliosis in a goat model. However, the mechanical strength of SME was insufficient to fully correct spinal deformities.
Pseudoelasticity, another property of nickel-titanium shape memory alloy, can be useful to overcome the above problem and provide a constant recovery force for deformations within the range of 8%. However, such constant correction forces must be kept low to avoid fracture at the anchoring points or interface of the bone and the implant and avoid neurological deficit.
The present invention provides a device and a method that overcomes the above problems. Additionally or alternatively, the present invention is capable of continuously providing a constant force to the spine even after surgery without post-operative manipulation and until the spine is fully or substantially corrected.